The present invention relates to a combustion grate, having a modular frame, and being installed in the furnace of a waste disposal plant, in which the aforesaid waste is burnt and so disposed of as ashes.
Such disposal generally comprises also an energy recovery system through the production of overheated steam and the exploitation of steam in a turbine, which in turn is coupled to an electric generator.
Such plants generally comprise a combustion chamber inside which the waste laid on a combustion grate is burnt, through which an adequate quantity of air is inputted. The combustion grate is adapted to sustain and advance the waste during the combustion permitting at the same time the insufflations of combustion air under the waste bed. The grate forms the lower portion of the combustion chamber. The combustion chamber begins physically immediately above the grate. In some cases, the walls of the combustion chamber are completely or partially cooled, by means of evaporation tube bundles protected by the refractory material itself. The region at the interface between grate and combustion chamber is made by the refractory-carrying beam. Inside the combustion chamber, the flame produced by the combustion of waste is spread, reaching temperatures over 1400° C. The surface of the grate is hit only occasionally by the radiation of the flame, as it is normally protected by the waste bed in transit.
The surface of the grate is made by plates (typically known as “fire bars”) which are normally made of molten steel having a high chromium content, in order to show high wear characteristics when hot. The advancement of waste is obtained through the relative movement of the fire bars which can have several characteristics. The actuation system is normally made of hydraulic pistons. The fire bars are provided with apertures or holes to allow the combustion air to flow from underneath the plane of the grate, through the waste. The combustion air has in fact the double function of providing the oxygen for the oxidation of the waste and of cooling the fire bar by maintaining it at an acceptable temperature in order to maintain the mechanical characteristics. The cooling is necessary, as the grates normally work covered by the forwarded fuel, but they can also be directly exposed to the combustion flames.
The steps making the grate can also be provided with an additional cooling system with water, particularly when they are used for the combustion of fuels with high calorific power. Such cooling is obtained by a liquid circulation which is forced to lap the surfaces which do not contact the fuel of each fire bar, through a liner or an equivalent apparatus for the accumulation of liquid.
The fire bars at the initial and terminal ends of each step are separated from the carrying structure of the grate by means of plates, generally of the same material of the fire bar, which are approached with pressure against the side of the side bar itself. Such plates have the function of laterally containing the fuel forwarded in the region immediately above the fire bars, and of separating it from the lateral portions of the grate, which do not tolerate the direct exposition to the burning material. The lateral plates eventually join the plane made by the fire bars with the vertical surfaces of the combustion chamber, which are disposed immediately above the grate.
The grate further comprises a plurality of handling groups, each formed by the cited fire bars organized in bundles, which relatively move one with respect to the other advancing the waste on the grate. In particular, the fire bars are divided into fixed and movable fire bars, which through slides cause a back-and-forth movement, by sliding one on another and determining the advancement of the waste in each handling group, and so in general on the grate.
The disposal of the solid urban waste, even if it is not characterized by a highest technological content, is an activity particularly sensible from a point of view of the reliability and the guarantees of the function. The complex integrated system of the waste disposal (the accumulation, the collection, the transport, the storing and the disposal) requires that the technological components employed in the last stage of the supply chain permit a continuous operation over hours and minimize the risks of stopping due to damages (minor or catastrophic damages as they can be). The operator of the disposal plant requires to the component “grate” to be reliable, robust and simple, both in the operation and in the maintenance stage.
Patent application MI2004A001746 describes a plant of this kind having a movable combustion grate, in which the handling group is formed by a plurality of fire bars or plates, which alternatively move one with respect to the other by advancing the waste on said grate, which realizes a substantially horizontal and at least partially continuous combustion plane.
The Applicant has observed that in such kind of technological solution the handling means of the fire bars, the sliding elements for the movable frame which supports the movable fire bars and the elements limiting the movement of the same, be in fact positioned in mutually remote regions of the grate, by making it necessary to dedicate to each of them an element separate from the carrying frame in order to support them. Furthermore, the elements limiting the movement are positioned along the symmetry axis of the module. This kind of assembly makes the rotary parts of the sliding and limiting elements particularly exposed to the unavoidable fall of ashes within the pressurized volume with the increase of the risk of malfunctions. Such positioning of the element makes further difficult the inspection, the maintenance and the lubrication of the rotary parts which must be reached by suitable tubing for the injection of the lubricant. Finally, the cited elements, the respective under-frames and the necessary constructions for supporting them impair the air flow which passes across the construction of the grate, by making difficult a correct distribution of the combustion air to the fire bars.